If said M3 is an E92, both stock I'd give the edge to the M3. Lighter, nearly the same power, geared more aggressively than the Camaro. With minor bolt-ons, I dunno as I trapped 111 bone stock and nearly 116 with cheap headers, a used CAI and generic tune.

If the M3 has the DCT transmission, they have the advantage. I had a buddy with an 11' M3 Manual. My L99 would rip him until 90mph, then he would start to pull away once he got into 7k + RPM's in 3rd gear. For some reason, he doesn't want to run me lately in my current ride and setup.